1. Field of the Invention
The present invention relates to an electron beam drawing apparatus for exposing a sample to a desired pattern or the like by irradiating the sample with an electron beam.
2. Related Art
In the field of optical communications, a highly precise diffraction grating is used for wavelength separations of a semiconductor laser or the like. Especially for multiple wavelengths in an optical communication fiber cable, in order to increase the transfer capacity, and investigations have been made against that increase. It is, therefore, an important subject to make the pitch size of the diffraction grating highly precisely. Specifically, it is necessary to draw the grating pitch of quarter microns in the precision of sub-nanometers.
The electron beam drawing apparatus has a higher resolution than that of an optical exposure apparatus or the like and can form a fine drawing pattern so that it is deemed suitable for the exposure of the fine diffraction grating. The demanded precision for the grating pitch is lower than the resolution of the size for the electron beam drawing apparatus to draw with an electron beam, thereby to make it very difficult to draw the grating pitch accurately. This difficulty is serious when a plurality of grating pitches have to be changed in the course of the drawing operation.
In the electron beam drawing apparatus, the drawing data are converted, after corrected in the deflection distortion, into an analog output by a DA converter thereby to deflect the beam. The problem is the resolution of the DA converter, i.e., the LSB (Least Significant Bit) value. The LSB value has to be a resolution higher than the sub-nanometer corresponding to the difference of the pitch which should be extremely finely variable. With the fine LSB value, the deflection range is restricted by the restriction on the bit number of the DA converter thereby to cause problems in the connections and the elongation of the drawing time. In the circuit, the pitch can be changed by adjusting the gain of the amplifier of the analog output circuit finely. However, it is necessary to complicate the circuit and to make the adjustment in a high grade.